In order to get high speed it is well-known that use of microcrystals rich in silver bromide is feasible. The intrinsic properties of AgCl/AgCl(I) emulsions are different from AgBr/AgBr(I) emulsions. As a consequence it is impossible to reach equal photochemical efficiencies for both systems without interfering in the intrinsic properties of the microcrystals rich in silver chloride. Comparing AgBr with AgCl from a point of view of differences between the energetic position of the highest valence level leads to striking conclusions. Especially for emulsions rich in silver chloride if used in combination with a green, a red or even with another dye absorbing at a higher wavelength, the quantum efficiency of the system may be limited because of the stability of the dye holes on the AgX surface, leading to recombination between photo-electrons and dye positive holes. This problem leads to "desensitization" and is known to be a factor limiting sensitivity, especially for high dye coverages. Enhancing the dye coverage of the emulsion crystals is nevertheless required in order to maximize absorption of light coming from light-emitting phosphors coated in the luminescent phosphor layers of intensifying screens. The advantage of making use therefore of {111} tabular crystals is related with the ratio of the surface to volume ratio of said crystals which is much higher than for globular crystals having an equivalent volume per crystal. It can thus also be expected that the dye desensitization will be more important for AgCl(I) {111} tabular crystals than for globular ones.
A classical way of preventing photocharges (photo-electrons and photo-holes) to recombine via a dye positive hole consists in introducing a reducing agent, sometimes called a supersensitizer.
However, this has been found to lead to severe shortcomings for the resulting materials. The adsorption of such a supersensitizer on the silver halide surface is contradictory to the idea of maximizing the adsorption of the spectral sensitizer and may dramatically limit the maximum dye coverage that can be attained. These organic dye molecules that should be added in a range of mmoles/mole of Ag may accumulate in the processing liquids thereby probably disturbing the optimum activity of these processings liquids or enhancing dye stain present after processing. Furthermore, introducing a reducing agent in an emulsion rich in silver chloride is rather critical for fog. In the concentration wherein a supersensitizer offers supersensitizing activity for green sensitizing dyes, fog can be formed either directly, either after a longer period, introducing stability problems. It is clear that there is a stringent demand for new measures in order to prevent the disadvantageous influences mentioned hereinbefore. Because of the high surface to volume ratio of {111} tabular silver chlor(oiod)ide microcrystals and as the stability of the morphology of the said microcrystals largely depends on the adsorption of grain growth stabilizers (such as the preferred "adenine"), whether or not in combination with spectral sensitizers, it will be very difficult to chemically sensitize these emulsions in order to get a limited number of efficient photo-electron trapping chemical sensitization centers per crystal. Concentration of the latent image in order to get less disperse latent image formation and a resulting increase in efficiency of silver utilization can be achieved by introducing specific dopants in the {111} tabular AgCl(I) crystals. This "latent image concentration effect" is known to depend on the specific position (site) of these dopants in the crystal with respect to the crystal surface and on the local concentration of these dopants in the doped phases or sites of the silver chor(oiod)ide microcrystals.
Adsorption of a large amount of organic compounds to the {111} tabular AgCl(I) crystal or grain surfaces will further strongly affect the developability of the material. The photochemical efficiency of the resulting material will therefore largely depend on the processing conditions, such as temperature, processing time, degree of oxidation/exhaustion of the processing solutions, etc . . .
In JP-A 03252649 and the corresponding USH 1294 and in JP-A 05134340, a detailed description of a material having tabular agcl(Br) emulsion crystals is given, for which a high sensitivity is claimed thanks to the introduction of dopants.
In U.S. Pat. No. 5,480,771 and similar patents as e.g. U.S. Pat. No. 5,500,335 the use of "SET" (Shallow Electron Trap)-dopants in combination with other kinds of dopants, thereby introducing deeper and more permanent traps for photo-electrons, are claimed. However the resulting materials have limited sensitivity and have very high contrasts because of the introduction of the other dopants. Therefore use of other "SET"'s like e.g. M(CN).sub.6-n L.sub.n or combinations of dopants with such sets is not suitable for radiological applications.
Similar effects on the developability can also be achieved by doping silver chlor(oiod)ide microcrystals with other kinds of dopants when added in minor amounts. E.g. when using a IrCl.sub.6.sup.3,4- compound, a non-permanent trap for photo-electrons is obtained in AgCl or AgCl(I) microcrystals wherein a characteristic trap time for the photo-electron in the center is observed in that it is much longer than in the so-called "Shallow Electron Traps" (SET's), as a result of the doping with proposed hexacyano metal ligand compound (see e.g. R. S. Eachus, M. T. Olm, Cryst. Latt. Def. and Amorph. Mat., 18, 297-313, (1989)).
It is known that at high concentration of dopants desensitization of the doped emulsion with respect to emulsions without dopant often occurs (see e.g. S. H. Ehrlich, I. H. Leubner, J. Imag. Sci. 36(2), 105, (1992).). Otherwise use of too small amounts of dopants has disadvantages, such as stability of the compounds in dilute solutions, effects of heterogeneous incorporation of the compounds over the crystal population, reproducibility, etc.
Moreover, the resulting traps are known to trap the photo-electrons as long as seconds or even longer, depending on the local halide composition of the crystal phase in which they are incorporated, on the presence of charge compensating vacancies and on the degree of aquation (see e.g. R. S. Eachus, M. T. Olm, J. Soc. Photogr. Sci. Technol. Japan, 54(3), 294, (1991)). As a result of the slow release of the trapped photo-electrons, the sensitivity of the materials will largely depend on the time between exposure and development. In the first minutes up to one hour after exposure a sensitivity increase may be observed (ref. H. Zwicky, J. Photogr. Sci. 33, 201, (1985)). Therefore in order to achieve a high sensitivity and a good developability without latent image instabilities in the first minutes after exposure SET's are incorporated as dopants and every improvement related therewith is highly requested.